I had a few good deals in local charity shops over the years. hardback 1st/1st's of the Trilogy books for £3 each (Gods Rincewind, Witches etc) the whole set. Just 2 weeks ago I found a completely mint Discworld puzzle for £2-99. Granted i spent nearly an hour counting the 1000 pieces before i bought it but it was worth it.
I only read the paperbacks and they are from the charity shops
I get all of my hardbacks online for the older ones, usually e-bay or Abe books. They are for the inheritence collection for Seren my daughter. The newer ones from Waterstones as the guys there are good friends and they usually go for £10 when first released. They have the limited slipcase editions most of the time so I get them all in one go. My O.C.D approach to collecting means that I have to have each different version. Snuff and Unseen Academicals was Slipcase Ed, Gold Edition/Hardback with Collector cards and Normal Blue 1st/Hardback without collector's cards. Totalling £50/£60 respectively. On this one I cheated a little. I couldn't wait for the paperback release so I spent many an awkward night
reading the one hardback through a 3" gap so not to break the spine!!! I dont drink or smoke so this is my little sin. I have to say I do love rummaging throught the local charity shop book racks. If I cant find a Pratchett i just spend my 50p on something else that looks good. Mo Hayder is my latest find. Very twisted but what writer. I've read Tokyo and Ritual in the last week. Pretty damn good